Organometallic compounds are used as a material for various purposes, such as transparent conductive oxides for use in fabricating photovoltaic cells and flat panel displays. Many organometallic compounds, such as diethyl zinc (DEZn), easily decompose and, in doing so, generate metallic compounds. In the case of DEZn, decomposition produces solid Zn and ethane/ethylene which, due to the difference in vapor pressure between ethane/ethylene and DEZn, tends to accumulate in the vapor region and increase the pressure in the storage container. The metallic compound gradually deposits in the storage tank, the supply equipment parts, and the filling lines during supply of the organometallic compounds to the manufacturing tool. This becomes problematic because the metallic compound not only contaminates the manufacturing process, but also causes stoppage of parts used in the supply system.
FIG. 1 is a diagram of a typical system that supplies a manufacturing tool 400 with an organometallic compound 211. To supply the organometallic compound 211 to manufacturing tool 400, a carrier gas 250 is introduced into the bubbler 210 through the carrier gas inlet line 251, the carrier gas inlet valve 252, and sparger 253, then the carrier gas 250 is dispersed in the organometallic compound 211 in the bubbler 210. The carrier gas 250 introduced in bubbler 210 becomes saturated with organometallic compound 211 and the saturated mixture is supplied to manufacturing tool 400 through the supply valve 242, the filter 243, the gas flow controller 244, and the supply lines 245 and 280. The supply equipment 200 includes the bubbler 210, the supply line 245, line 233, and the parts located on line 245 and line 233, for example, the gas mass flow controller 244 and the filter 243. The supply line 280 is the pipe between the supply equipment 200 and the manufacturing tool 400, denoted by the arrows. Supply line 280 may also have parts located thereon, for example, valves, connections, gas flow controllers, gas flow meters, filters, etc. (not shown). The refill line 180 is the pipe between the supply equipment 200 and the filling valve 142 installed on the storage tank 110 located in the refilling equipment 100, which is also denoted by arrows. The refill line 180 may also contain parts, such as a liquid mass flow controller 144 etc.
Keeping the level of organometallic compound 211 constant in the supply equipment bubbler 210 is possible thanks to refilling equipment 100 even during usage of the organometallic compound 211 in the bubbler 210. The organometallic compound 211 may be used continuously without emptying the bubbler 210. The storage tank 110 mentioned above fills liquid organometallic compound 111 into the bubbler 210. To fill the bubbler 210 with the organometallic compound 111, a carrier gas 150 is introduced into the storage tank 110 through the carrier gas inlet line 151 and the carrier gas inlet valve 152, and the storage tank 110 is pressurized. The organometallic compound 111 is then transported through the siphon tube 141, the filling valve 142, the filling line 143, the liquid mass flow controller 144, the supply equipment line 233, and the filling valve 232, filling the bubbler 210 with the compound 111.
As the storage tank 110 becomes empty, the tank 110 is sent to a chemical maker. The continuous supply of organometallic compound to the bubbler 210 is maintained by providing another storage tank 110. The metallic compound (not shown) deposited on the tank 110 is removed by the chemical maker regularly before the tank 110 is filled with new or fresh organometallic compound 111. The storage tank 110 filled with new organometallic compound 111 may then be connected to the supply equipment 200 and used again.
The storage tanks used in the semiconductor industry or the photovoltaic industry are typically made of steel, for example stainless steel. Because the metallic compound deposited in the storage tank is difficult to dissolve in most solvents, a strongly corrosive acid solution, such as a hydrofluoric acid or nitric acid solution, is typically used as the cleaning solvent prior to filling the storage tank with fresh organometallic compound. Cleaning the storage tank of the metallic compound that has deposited on it is associated with several difficulties. Many organometallic compounds, such as DEZn, react violently with water and therefore any residual DEZn that remains in the tank may react with water in the hydrofluoric or nitric acid solution. The violent reaction may create hazardous conditions that must be controlled.
A second issue related to the use of a hydrofluoric or nitric acid solution is the attack of the acid on the material of which the storage tank is comprised. Strong acids will corrode steels, and therefore the exposure time should be minimized to limit any negative impact on the steel material. Therefore, control of the cleaning process, acid concentration, and acid cleaning time is essential when cleaning the decomposed metallic compound from stainless steel storage tanks to avoid corrosion. Rinsing the storage tank with pure water for a long time to remove the remaining acid is also necessary to prevent the tank from corrosion after acid cleaning. Moreover, purging the storage tank with nitrogen for a long time is necessary to dry the tank after the pure water rinse to avoid causing a violent reaction between the organometallic compound such as DEZn and any residual water in the storage tank.
Selectively cleaning the metallic compounds deposited on a device made of steel without corrosion of the device is difficult. Therefore, accurate control of the acid concentration and the acid cleaning time are necessary to avoid corroding the device. As a result, cleaning any device (e.g. storage tank, valve, tubing, flow controller etc.) using a classical acidic solution such as hydrofluouric or nitric acid is a complex process because adequate acid exposure time must be ensured to remove the decomposed metallic compound without damaging the materials of which the device is comprised and ensuring a process so that the organometallic compound never comes in contact with water to avoid any potentially violent reaction. As a result, the cleaning process using acidic solution has many steps and as a result is long and costly.
The reason that it takes a long time to clean the storage tank and that the ordinary cleaning process requires accurate control is that the acid solution has a substantial amount of water in it (>50% H2O by weight) and water reacts violently with many organometallic compounds, such as DEZn. Nevertheless, acid solutions have typically been used as the cleaning solution for stainless steel storage tanks or other devices, even though the acid has corrosive properties against steel, as effective alternative solvents have not been identified or used in the industry. The usage of other types of cleaning solutions, for example those containing surfactants, have not been used because these solutions typically contain atoms such as sodium or potassium, which are contaminants that negatively affect the performance of semiconductor devices and solar cells. The acid solution is widely used due to the above-mentioned reasons. However, when the acid solution is used as a cleaning solvent, it is necessary to control the concentration of the acid accurately, and to manage the acid cleaning time accurately, resulting in a complicated cleaning process.
After the storage tank is cleaned by the acid solution, a pure water rinse of the tank is necessary for an extended time period (several minutes to hours) in order to remove the acid from the storage tank because the tank may corrode if any small amount of acid remains. Moreover, the tank then requires a nitrogen purge for an extended period of time (minutes to hours, but typically longer than the pure water rinse time), requiring a large amount of nitrogen to dry the tank after the pure water rinse.
Considerable caution and a skilled technique are needed to clean a tank that was used for the compounds having high reactivity with water because the acid solution contains water. Therefore, cleaning solvents and cleaning methods capable of cleaning the storage tank easily and safely are needed.
On the other hand, the supply equipment parts, such as the supply lines or the filling lines, are not cleaned regularly like the storage tank. When the metallic compound is deposited on the equipment parts that supply the manufacturing tool with the organometallic compound, there are two commonly employed solutions. The first is to clean the part after disconnecting it from the organometallic compound supply system. A nitrogen purge of the part is needed before the part, as well as a nitrogen purge and leak check after connecting. This solution takes time, personnel cost, and cleaning cost.
The second solution is to replace the part with a new part. A nitrogen purge of the part is needed before replacing, as well as a purge and leak check after replacing. This solution also takes time, personnel cost, and the cost of new part. The supply line may need to be replaced because the length of the supply pipe may be many meters long, frequently about 30 m, and therefore provides a large surface area on which the metal may deposit. An improvement to the existing two solutions would be to clean the parts in place, without disassembling the parts. This is not done in practice today because the most widely used cleaning solution is an acidic solution which may react with any residual DEZn in the part or line.
In many cases when the metallic compound deposits on the equipment parts (such as the supply line and the filling line), the parts have to be detached from the supply system and then cleaned by acid solution or exchanged for a new part. When an acid solution is used as the cleaning solvent, accurate control of the process is necessary and considerable caution and a skilled technique are needed for organometallic compounds that are highly reactive with water, as mentioned above. In addition, when the parts are cleaned after detaching from the supply system, time and personnel cost for the nitrogen purge and leak check are needed, as well as the cost of the cleaning. Detaching and cleaning of long length pipes is difficult and frequently requires replacement by a new pipe.
Cleaning solvents and cleaning methods that easily and safely clean metallic compounds deposited on the equipment parts (e.g., the supply line, the filter, the filling line) used in the semiconductor industry or the photovoltaic industry without detaching the part from the supply system are needed.